A hot press method or a die quenching method is one of manufacturing methods for parts such as a part for a vehicle that requires high strength. The manufacturing method is composed of heating a steel workpiece up to 900 degree C., for example, and press-forming or quenching at the same time of the press-forming to harden the workpiece.
A heating device such as a furnace is usually used for heating a workpiece. However, it takes approximately three to five minutes to heat a workpiece up to 900 degree C. in a furnace and the heating time is rather longer than the time for a press-forming step. In addition, because it takes some more time to take out the workpiece from the furnace and transfer it to the press-forming step, it causes temperature drop, unevenness of temperature and generating scales.
Thus an electric heating (conduction heating) is used to solve the problem. This is a method to heat using the Joule heat generated by a large electric current across electrodes attached on both ends of a workpiece. In this case, semi-spherical electrodes or flat bar electrodes, which contact with the workpiece flatly, are used. Patent Document 1, for example, discloses one or more semi-spherical electrodes contacting with each end portion of a metal plate.
The electrodes are connected with both end portions of a workpiece and when the electrodes are flat bar electrodes, the electrodes should be sufficiently contacted with the workpiece. For this purpose, one end of the workpiece is sandwiched from both sides (upper surface and opposite surface) by a pair of electrodes confronting each other (each of which is called as upper electrode and lower electrode, respectively) and is clamp-held, and both ends are similarly clamped and contacted enough, and then electric current is fed across the electrodes on both ends.    Patent Document 1: JP Patent Kokai Publication No. JP-P2002-18531A